1. Field of Invention
This invention relates generally to the copolymerization of olefin monomers and in particular to the formation of olefin-containing tapered block polymers.
2. Related Art
Gradient, or tapered, copolymers have a chemical structure with a gradual change of composition along the polymer chain from one monomer unit to another that is intermediate between block- and alternating-copolymer structures (1,2). FIG. 1 shows schematically the arrangement of different copolymers, including gradient copolymers, with open and closed circles representing two different monomer units. The gradient structures can be tuned to combine the properties of the different homopolymers. If the average chain length is sufficiently long, the degree of incompatibility between the two monomer units high, and the gradient profile provides chain ends that contain mainly one type of monomer unit, then microphase separated structures similar to those observed in traditional block copolymers can be observed. These special types of gradient copolymers are called tapered block copolymers and are technically relevant since they can be tailored to be efficient compatibilizers of polymer blends and can be superior to block copolymers for this application (3, 4, 5). The phase morphology and interfacial energy in tapered block copolymers is modulated by the length and sharpness of the composition gradient (6-10)
Living polymerization methods are required for the synthesis of gradient structures, since to achieve similar average composition drifts in each polymer chain, all chains must be initiated simultaneously and must participate in the propagation sequence until the end of the reaction (2). Because of this requirement, gradient copolymers have been synthesized primarily by controlled radical polymerization (11-15) and anionic polymerization methods (16-23). Tapered block copolymers derived from ethylene and functionalized comonomers have not been reported, primarily because of the limited options for living polymerization initiators (24-29). Such polymers can make a substantial impact in designing blends of polyethylene and polar engineering plastics because the polarity gradient along their chain would reduce the interfacial energy and lead to better mixing. Additional applications include, for example, use as high temperature adhesives and as coatings for polar substrates.
There are seven accepted criteria for living polymerizations with living catalyst systems. These are: 1) the polymerization proceeds to a complete monomer conversion and restarts upon further addition of the monomer; 2) linear dependence of Mn with time; 3) the number of active sites remains constant during polymerization; 4) the molecular weight can be precisely controlled by stoichiometry; 5) a narrow PDI; 6) sequential monomer addition results in a block copolymer; and 7) an end-functionalized copolymer can be synthesized. Systems that partially fulfill this list of criteria are termed “quasi-living”. Quasi-living catalysts for ethylene and norbornene copolymerization have been described (30). However, no functionalized norbornene derivatives were copolymerized, and no tapered block copolymers were described.
Catalysts for quasi-living polymerization of olefin monomers are disclosed in U.S. application Ser. Nos. 10/377,491 now U.S. Pat. No. 7,259,214 and 10/378,957, both prior applications of one of the present inventors and herein incorporated by reference. The catalysts are formed from a combination of two neutral metal complexes (31, 32). These catalysts use Group VIII transition metals rather than the Group IV transition metals more commonly used in catalysts. Compared to the early transition metals, the late transition metals are more tolerant of olefin monomers with functional groups.
Olefin-containing tapered block copolymers have application as thermoplastic elastomers, adhesives, polymer coatings, oxygen barrier films, and polymer blend compatibilizers, as well as other uses. However, few methods are available for synthesizing such polymers. The development of new methods of olefin copolymerization are required to take full advantage of the uses offered by olefin-containing tapered block copolymers.